1. Field of the Invention
The invention relates generally to bandgap reference circuits, and more particularly to bandgap reference circuits with reduced power consumption.
2. Description of the Related Art
One of the essential building blocks of many analog circuits is a voltage reference, which is configured to exhibit little dependence on supply and process parameters and a well-defined dependence on temperature. Accurate biasing voltages are critical for many circuit schemes. For example, in an analog-to-digital converter (ADC), a reference voltage is required to accurately quantify an input, while in a digital-to-analog converter (DAC), the reference voltage is required to define the output full-scale range.
Bandgap reference circuits are conventionally used to maintain the voltage reference at a predetermined level. The general principle of bandgap reference circuits relies on two diode-connected BJT transistors (or junction diodes 105 and 110 as illustrated in FIG. 1) running at different emitter current densities. By canceling the negative temperature dependence of the PN junctions in one group of transistors with the positive temperature dependence from a proportional-to-absolute-temperature (PTAT) circuit which includes the other group of transistors, a fixed DC voltage that does not change substantially with temperature is generated.
FIG. 1 illustrates a conventional bandgap reference circuit 100. Referring to FIG. 1, the bandgap reference circuit 100 includes PMOS transistors M1, M2 and M3, an operational amplifier 105, resistors R and kR, and diodes 110, 115 and 120. The operational amplifier 105 functions to equate the voltages V1 and V2 and generate a PTAT voltage across the resistor R, as shown in FIG. 1. The output of operational amplifier 105 drives the gates of transistors M1, M2 and M3, to generate the current I_ptat having a positive temperature dependence, due to the different current densities in the PN junctions of diodes 110, and 115. The positive temperature dependence of I_ptat can be used with the negative temperature dependence of the PN junction of diode 120 to generate the temperature independent bandgap reference voltage (Vbg), as is known in the art.
If the operational amplifier 105 was an ideal component V1 would equal V2. However, the operational amplifier 105 also amplifies the input-referred noise to the output voltage, or bandgap voltage Vbg. Likewise, similar to the input-referred noise, the input-referred offset voltage of the operational amplifier 105 also gets amplified and affects the bandgap voltage Vbg.
Generally, in the bandgap reference circuit 100 of FIG. 1, the burden of maintaining the low overall amount of noise in the bandgap voltage Vbg is placed on the operational amplifier 105. Thus, the operational amplifier consumes a relatively high amount of power in order to maintain noise at acceptable levels.
Ideally, the output voltage of a bandgap reference circuit should be substantially constant irrespective of Process, Voltage, and Temperature (PVT) variations. As discussed above, bandgap reference circuit design conventionally focuses mainly on temperature compensation. However, process variations may have the biggest impact on the absolute value of the reference voltage. For example, in the circuit illustrated in FIG. 1, the input offset voltage of the operational amplifier 105 may vary considerably due to process variations in the material and manufacture that are present in any large scale production of integrated circuits (e.g., millions of units). As noted above, this input offset voltage gets amplified and will create an error in the bandgap voltage Vbg.